The present invention relates to a method for alkane (amm)oxidation to unsaturated carboxylic acids or nitriles using a mixed metal oxide (MMO) catalyst. Specifically, it relates to a method for removing sulfur impurities in the feed and a method for the selective removal of reduced forms of sulfur impurities. More specifically, it relates to the removal of sulfur impurities from propane or a propane oxidation feed that is oxidized to acrylic acid.

As used herein, the use of the term "(meth)acrylate" refers to both acrylates and methacrylates. Similarly, the term "(meth)acrylic" refers to either acrylic or methacrylic; the term "(meth)acrylic acid" refers to either acrylic acid or methacrylic acid.

As used herein the term (amm)oxidizing is meant to refer to both the processes in which mixtures of propane, ammonia, and oxygen are converted in the presence of a catalyst to a nitrile as the primary product and in which mixtures of propane, and oxygen are converted in the presence of a catalyst to a carboxylic acid as a product.

Acrylic acid (AA), one example of an unsaturated carboxylic acid, and acrylonitrile, one example of a nitrile are used in a wide variety of applications. Typical end-use applications include: acrylic plastic sheeting; molding resins; polyvinyl chloride modifiers; processing aids; acrylic lacquers; paints and coatings; building products; floor polishes; sealants; auto transmission fluids; crankcase oil modifiers; automotive coatings; ion exchange resins; cement modifiers; water treatment polymers; electronic adhesives; metal coatings; and acrylic fibers. AA is especially prized in these applications and others because of the hardness it imparts to the products in which it is used. It also enhances chemical stability and light stability, as well as ultraviolet radiation resistance, when used in certain products. Therefore, AA is often used in applications requiring resins of excellent transparency, strength, and outdoor durability. The AA market is extremely cost-sensitive; thus, any improvement in process yield, however slight, can result in significant cost-savings.

Commercial grades of propane, which may be employed as a feedstock for the production of unsaturated carboxylic acids and nitriles, typically contain a number of sulfur-bearing compounds including, but not limited to, mercaptans (RSH, namely methyl and ethyl), sulfur dioxide (SO₂), carbonyl sulfide (COS), hydrogen sulfide (H₂S), sulfides (RSR), and disulfides (RSSR), where R is an alkyl or aryl group. Typical specification levels for commercial grade range from 100 to 150 ppm by weight of total elemental sulfur.

The adsorption and reaction of these sulfur compounds on the mixed metal oxide catalysts useful for alkane (amm)oxidation can be deleterious to catalyst performance and stability. Of particular concern is a loss of long-term performance, especially long-term catalyst stability. The adsorption onto and subsequent reaction of reducing forms of sulfur compounds, such as hydrogen sulfide, with an MMO alkane oxidation catalyst may reduce the oxidation state of the catalyst's constituent catalytic metals. Thus, the redox properties, which are crucial to the catalytic properties of these MMOs, may be affected, and the presence of these reducing compounds in the alkane oxidation feed may therefore deteriorate the catalyst's performance and/or stability, resulting in a state known in the art as "sulfur poisoning." Low levels of oxidizing forms of sulfur in the (amm)oxidation feed are not deleterious to MMO catalysts and can in fact improve catalyst lifetime and enhance the (amm)oxidation yield. The MMO catalyst performance is also improved when the catalyst is prepared to contain sulfur oxides as promoters. Thus, a need exists for the selective removal of the reducing forms of sulfur from the alkane (amm)oxidation feeds.

Japanese published patent application 2001-342169 discloses a method of producing acrylic acid and/or acrylonitrile containing lower amounts of sulfur impurities by an adsorption process. The reaction is conducted using a catalyst containing Mo, V, and Te or Sb as required elements, from propane wherein the total feed gas contains less than 200 ppm sulfur. The application discloses the use of zinc oxide, activated carbon, copper oxide, iron oxide, ruthenium, nickel, palladium, or aluminum desulfurizing adsorbents for eliminating oxidizing and reducing forms of sulfur compounds such as H₂S, COS, carbon disulfide, SO₂, RSH, RSR, RSSR, and thiophene.

For the commercial production of acrylic acid or acrylonitrile by propane (amm)oxidation, the use of the desulfurizing adsorbents disclosed in JP 2001-342169 presents a problem in that these materials are consumed over time as they accumulate or react with the sulfur impurities. They must therefore be periodically regenerated or replaced, hence the sulfur removal itself is conducted in a batch or semi-continuous operation. Regeneration or replacement of the adsorbent becomes cumbersome and expensive for large scale continuous processes. Thus, the need for processes for producing (meth)acrylic acid or (meth)acrylonitriles from alkanes by processes in which sulfur is removed continuously are needed.

Additionally, in order for methods of producing (meth)acrylic acid or (meth)acrylonitrile from alkanes to benefit from oxidized forms of sulfur present in the alkane oxidation feed, methods in which reduced forms of sulfur are selectively removed are needed.

The improved alkane (amm)oxidation process of the present invention avoids the problem of sulfur poisoning and increases process efficiency by providing for the integration of continuous sulfur removal techniques with the alkane (amm)oxidation technology known in the present art and provides a method for the selective removal of sulfur from alkane (amm)oxidation reactions.

Effective sulfur removal techniques of the present invention largely comprise, absorption unit operations. Absorption techniques subject the feedstock to a liquid absorbent; the sulfur impurity is retained by chemical reaction or solvation with the absorbent.

In the present invention, there is provided, a process for (amm)oxidizing alkanes to unsaturated carboxylic acids or nitriles comprising:

1. i. reacting an alkane with oxygen, oxygen and ammonia, or an oxygen and ammonia containing gas in the presence of an oxidation catalyst to yield a product comprising at least one unsaturated carboxylic acid or nitrile and a concentration of saturated carboxylic acid or nitrile comprising a sulfur impurity concentration; and
2. ii. lowering the sulfur impurity concentration by a continuous absorption process.

The method comprises reacting in a single reaction vessel at least one C₃ - C₈ straight chain or branched alkane. As used herein, the term "C₃ - C₈ straight chain or branched alkane" means a straight chain or branched chain alkane, although longer alkanes may also be used, having from 3 to 8 carbons atoms per alkane molecule, for example, propane, butane and pentane. Particularly, C₃ - C₅ straight chain or branched alkanes are examples of alkanes of the present invention.

When the alkane comprises propane, the unsaturated carboxylic acid comprises acrylic acid, and the nitrile comprises acrylonitrile. The alkane is reacted with oxygen or oxygen present in a mixture with an inert gas, such as nitrogen, in the presence of a catalyst at a temperature ranging from upper limits of 550 °C, 480 °C, and 400 °C to lower limits of 250 °C, 275 °C, and 300 °C. All temperature ranges are inclusive and combinable.

In one embodiment, the catalyst comprises a mixed metal oxide (MMO) having the empirical formula

         AaMbNcXdZeOf

wherein A is at least one element selected from the group consisting of Mo and W; M is at least one element selected from the group consisting of V and Ce; N is at least one element selected from the group consisting of Te, Sb and Se; X is at least one element selected from the group consisting of Nb, Ta, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Sb, Bi, B, In, As, Ge, Sn, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Hf, Pb, P, Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu; and Z is at least one element selected from the group consisting of Zn, Ga, Ir, Sm, Pd, Au, Ag, Cu, Sc, Y, Pr, Nd and Tb; and wherein, when a = 1, b = 0.01 to 1.0, c = 0.01 to 1.0, d = 0.01 to 1.0, e = 0 to 0.1, and f is dependent on the oxidation state of the other elements.

The reactants are admixed or otherwise provided with a diluent. The diluent is preferably a gas at room temperature and ambient pressure; and is inert to the reaction environment under the existing reaction conditions. Suitable gases include nitrogen, argon, helium and the like. The amount of diluent is not particularly important, however, it is preferably present in an amount of from greater than 0.1 mole percent to less than 70 mole percent, based on the total feed to the reactor, when used. Also, steam may be present in the feed gas in an amount varying from zero to 50 percent.

Suitable reaction vessels are designed to conduct vapor-phase, heterogeneous reactions and include but are not limited to fixed bed, fluidized bed, plate and frame, and micro-channel reactors. Where propane is used as the starting alkane and air is used as the oxygen source, the reaction vessel may be a fixed bed system or a fluidized bed system. The proportion of air to be supplied to the reaction system is important for the selectivity for the resulting acrylic acid or nitrile. For acrylic acid, the proportion of air supplied is usually at most 25 moles, but usually ranges from 0.2 to 18 moles per mole of propane, whereby high selectivity for acrylic acid can be obtained. This reaction can be conducted under atmospheric pressure. However, the reaction may also be conducted under a slightly elevated pressure or slightly reduced pressure. With respect to other alkanes, for example isobutane, the composition of the feed gas may be selected in accordance with the conditions for propane.

Typical reaction conditions for the oxidation of propane to acrylic acid or methacrylic acid or acrylonitrile may be utilized in the practice of the present invention. The process may be practiced in a single pass mode where only fresh feed is fed to the reactor, or in a recycle mode where at least a portion of the reactor effluent is returned to the reactor. General conditions for the process of the present invention are as follows: the reaction temperature ranges from upper limits of 550° C., 480° C., 400°C. to lower limits of 250° C., 275° C., and 300° C: to the gas space velocity, "SV", in the vapor phase reactor is typically within a range from upper limits of 10,000, 6,000, and 2,000 hr-1 to lower limits of 300, 200, and 100 hr- 1; the average contact time with the catalyst can range from upper limits of 10, 8, 6, and 2 seconds to lower limits of 0.01, .01, .5, and 1 seconds; and the pressure in the reaction zone usually ranges from 0 to 75 psig, but is typically no more than 50 psig. All of the aforementioned range values are inclusive and combinable within a given set. In a single pass mode process, it is preferred that the oxygen be supplied from an oxygen- containing gas such as air. The single pass mode process may also be practiced with oxygen addition. In the practice of the recycle mode process, oxygen gas by itself is the preferred source so as to avoid the build up of inert gases in the reaction zone.

As a result of reacting at least one alkane with oxygen, oxygen and ammonia, or an oxygen and ammonia containing gas, such as air, in the presence of at least one catalyst, a product is formed that comprises a least one unsaturated carboxylic acid or nitrile and a concentration of a saturated carboxylic acid or nitrile containing at least one sulfur impurity. As used herein by "sulfur impurity" is meant H₂S, COS, carbon disulfide, SO₂, RSH, RSR, RSSR, and thiophene.

In one embodiment, these sulfur compounds are removed by physical absorption. The sulfur compounds are removed by contacting the alkane or (amm)oxidation feed gas with an absorbent in an absorption zone. As used herein "absorption zone" is defined as the physical place in the reaction process where the sulfur compounds are absorbed into the absorbent. The absorbent containing the sulfur compound is then cycled continuously to a desorption zone, similarly, where the absorbed sulfur compounds are removed, for example, by flash evaporation or stripping. The absorbent is continuously collected from the desorption zone and returned to the absorption zone; the sulfur compounds collected in the desorption zone can then be subjected to disposal. Suitable physical absorbents for sulfur compounds include, but are not limited to, methanol, dimethylethers of polyethylene glycols, propylene carbonate, sulfolane, and 1-acetylmorpholine. A non limiting example of a typical physical absorption process is pressure swing absorption.

In another embodiment, sulfur compounds are removed by chemical absorption, which operates similar to the physical absorption process in that the sulfur compounds are absorbed in an absorption zone and desorbed in a desorption zone. The key difference is that the sulfur compounds chemically interact with the absorbent by reversible chemical reactions or complexation. Suitable chemical absorbents for sulfur compounds include aqueous organic amines, for example, monoethanolamine, diethanolamine, diglycolamine, and methyldiethanolamine and aqueous alkali carbonates, for example, potassium carbonate. An example of a commercial chemical absorption process is Petrolite's Sulfa-scrub HSW process in which sulfur is removed from natural gas by absorption into aqueous triazines.